The ability to localize sound is integral to communication and environmental awareness. Normal hearing listeners localize sound in the horizontal plane by responding to interaural differences in signal timing (ITD) and level (ILD) at the two ears. In ordinary listening environments (e.g., rooms), accurate localization requires that listeners respond to the cues carried by the early-arriving incident sound rather than the spurious cues carried by later-arriving reflected sound. In fact, localization by normal hearing listeners is remarkably unaffected by echoes and reverberation. The various phenomena associated with this observation are known collectively as the precedence effect. Although the adaptive value of the precedence effect in sound localization is clear, important questions about the effect remain. For example, although ITD and ILD are distinct acoustic cues, thought to be processed separately in the auditory brainstem, most precedence findings reflect data from studies employing headphone ITD or free field stimuli where the contributions of ITD and ILD to localization judgments are inseparable: relatively little is known about the contribution of ILD to precedence, despite evidence that ILD is the dominant cue in localization for some listeners, including a growing population of bilateral cochlear implant users. Thus, the broad long-term goal of the proposed research is to elucidate the separate contributions of ITD and ILD to the precedence effect in order to establish a more complete and clinically informative account of sound localization in ordinary listening environments. The specific aims of the proposed research are (1) to examine listeners'sensitivity to "echoes" following repeated presentations of "source-echo" stimuli carrying only ITD or ILD and (2) to examine listeners'sensitivity to "echoes" following repeated presentations of "source-echo" stimuli carrying combinations of ITD and ILD. Recent data from the Applicant's lab suggests that the time course of ITD sensitivity is substantially different from the time course of ILD sensitivity. Thus, toward the first aim, an initial experiment will test the hypothesis that echo suppression is more robust for stimuli carrying ITD than for stimuli carrying ILD. Toward the second aim, an additional experiment will test the hypothesis that "dynamic" echo suppression depends on sensitivity to both ITD and ILD "images." Across experiments, stimuli will be presented via headphones to allow for independent manipulation of ITD and ILD cues, and echo thresholds will be estimated by an adaptive psychophysical procedure. The proposed research will provide valuable new insight on a large body of existing precedence effect literature and on the unique roles of ITD and ILD in spatial hearing. The work will thus inform both basic and clinical understanding of audition, underscoring this proposal's relevance to the mission of NIDCD. PUBLIC HEALTH RELEVANCE: The ability to determine where sounds are coming from is an important part of hearing. Although everyday listening environments introduce echoes and reverberation that might be expected to make it very difficult to determine the location of actual sound sources, normal hearing listeners are remarkably capable of accurate localization under such conditions. The proposed research builds on what is known about this capacity to expand basic understanding of sound localization and to provide insight on the reduced sound localization abilities of certain hearing impaired populations, including bilateral cochlear implant users.